LeechCraft is a cross-platform modular "internet-client" application.
It has plugins that allow one to browse the web, chat via instant messaging (like Jabber or IRC), read RSS/Atom feeds, download files via BitTorrent, Direct Connect and other protocols, play media files and much more.

BitDHT is a LGPL'd general purpose C++ Distributed Hash Table library. It is designed to take hassle out over creating your own DHT. BitDHT is compatible with bitttorrent's DHT and can leverage this network to bootstrap your own personal DHT.

TorrentVolve is a cross-platform PHP-driven web-based BitTorrent client. It focuses on speed and reliability, while also providing a full feature set. It includes a user management system, a Torrent file manager, and a configurable Torrent downloader.

An PHP + MySQL script that attempts to recreate features similar to the likes of Mininova, SeedPeer, and TPB frontend. Features a new design, crawling ability, cURL for single torrent scrapes + mass scrapes, and much much more! Based on T-Xore 0.4, Ibitz

XBNBT is a modifed BNBT BitTorrent tracker written in C++. XBNBT offers many additional features beyond the original BNBT tracker and has been designed with the Linux user in mind in an attempt to make it as easy as it is for Windows.

Transparent BitTorrent -- Browse with Firefox and download large files normally. Transporrent monitors your download and if it slows down, transparently switches to using BitTorrent in the background. Transporrent requires a browser Add-on and server mod

DigitalBicycle is a web community for independent media producers, New Media Collectives, Community Media and Technology Centers,and other Local Access Points using BitTorrent, RSS, and Drupal.
See digitalbicycle.org for more information.

Leonidas is basically the ASP.NET porting of the phpMyBitTorrent Tracker. Written in both C# and Visual Basic and fully compatible with Apache/Mono, this tracker offers many advanced features for BitTorrent community.